Most automobiles are still manufactured with an electrical system of conventional type, where separate electrical conductors are taken from lamps and other audiovisual means as well as servo motors and solenoids and the like, to the automatic and manual indicators and control means that are to control the lamps, servo motors, etc. However, there are described in patent publications several time multiplex systems intended for entirely or partially replacing conventional electrical systems for current supply and control of different components included in the equipment of the vehicle. Examples of publications where such systems are described include U.S. Pat. Nos. 3,651,454; 3,891,965; 4,015,136; 4,015,136; 4,370,561; 4,578,591; 4,584,487and No. 4,594,571.
The configuration of conventional electrical systems for vehicles make them suitable for transmission of analogue information, e.g. coolant temperature and fuel quantity as well as digital information such as ON/OFF. Although it has been proposed to use time multiplex systems for trasmission of both analogue and digital information, as in U.S. Pat. Nos. 3,651,454, 3,891,965 and No. 4,015,136, for example, many time multiplex systems for vehicles are not particularly suitable for transmitting both analogue and digital information.
Conventional electrical systems for automobiles are often formed such that certain components, e.g. parking lights, can be used irrespective of whether the vehicle is being driven or whether it is parked and locked. Conventional electical systems are also usually formed so that certain components, e.g. a car radio, can be used only if the ignition is unlocked. On the other hand, it is not necessary for the ignition to be switched on. Finally, conventional electrical systems are often formed such that some components with high power consumption e.g. headlight low beams can only be switched on when the ignition is switched on, or when the generator is charging. The purpose of this is to prevent the battery from becoming discharged too quickly, e.g. by the low beams being left switched on when the vehicle is not in use. At the same time the components usually need to be used in a given situation are useable in that situation and the built-in safety against too-rapid discharge of the battery is usually not experienced as irritating or disturbing by the person using the vehicle.
Conventional electrical systems can be easily formed so that in principle they do not consume any power when the vehicle is not used, but in spite of this they have a facility of controlling certain components even before the vehicle ignition is switched on. A conventional electrical system with separate conductors can be formed, for example so that when the vehicle is locked and parked its central door lock can be operated by the door key, in spite of the electrical system not consuming any notable power before the central lock is operated by the door key.
Most of the proposed time multiplex systems for auotmobiles do not have the above-mentioned properties. Power supply to the multiplex system is either ON, the multiplex system then having the maximum capacity for transmitting information, or the power supply to the multiplex system if OFF, the system then completely lacking capacity of transmitting information. When the power suppy to the multiplex system is ON, which can be arranged via the ignition lock or a separate means, the multiplex system has a power consumption which is not completely negligible.
A system is described in the U.S. Pat. No. 3,864,578 for transmitting information in time multiplex in vehicles, this system being formed to enable the assumption of an active mode with comparatively high power consumption, or a standby mode with comparatively low power consumption. When this system is in active mode it has maximum capacity for transmitting information in the vehicle. From its active state the system enters into its standby mode of operation when neither the vehicle ignition is ON nor any of a number of "critical" operating means are actuated. In addition, when the system is in its standby mode, it only has the ability of transmitting information enabling the system to enter into its active mode, which occurs when the vehicle ignition is switched on or when any of a number of "critical" operating means is actuated.
The system according to U.S. Pat. No. 3,864,578 includes encoding and decoding means connected to three lines. One of the lines is utilized for power supply to the means and is connected to a battery. Both the other lines are then utilized for transmission of timing signals and information. When the system is in its active mode all the encoding and decoding means are in an active mode as well. In its active mode, a timing encoding means sends a clock signal on one of the lines and a synchronizing signal on the other line. The clock signal has the form of a square wave superposed on a DC voltage. The synchronizing signal has the form of individual pulses superposed on a DC voltage. The clock signal and synchronizing signal together define a plurality of time slots for time multiplex transmission of information. The clock signal and synchronizing signal are received by all the other encoding and decoding means, which are sensitive to the clock signal and are kept in an active mode by it. In an active mode, the encoding and decoding means can send and receive information in the time slots. The transmission of information in the time slots takes place on the same line as the synchronizing signal and in the form of a binary pulse per time slot. The binary pulses are thus trasmitted superposed on a DC voltage. When this system is in its standby mode, both the time controlling encoding means and the remaining encoding and decoding means are in their standby modes. The time controlling encoding means sends neither the clock signal nor the synchronizing signal. On the other hand, battery voltage is available at all encoding and decoding means. When a "critical" operating means connected to one of the remaining encoding and decoding means is actuated, this means provides in its standby mode a DC voltage on the line used for the synchronizing signal. The timing encoding means in its standby mode is sensitive to the potential on this line and is caused by the voltage to assume its active mode and begins to send the clock signal and synchronizing signal. The remaining encoding and decoding means are sensitive to the clock signal in their standby modes and are all caused by it to enter into their active states. Information corresponding to the actuation of the "critical" operating means can thus be transmitted by the system.
Conventional electrical systems for automobiles, apart from battery and generator, have in general no particular vehicle component with a function vital to the whole electrical system. Conventional electrical systems with separate electrical conductors and fuses for a plurality of vehicle components are usually formed such that the main part of the electrical system can function although a part of it has collapsed. Proposed systems for time multiplex transmission for information between vehicle components do not usually have this property. They usually have a central timing or synchronizing means, the function of which is completely decisive for the function of the entire system. If the timing or synchronizing means ceases to send signals, the whole system ceases to function.